1. Field of the Invention
The present invention relates to noise filters for use in electronic circuits utilizing differential signals, such as in high-speed differential interfaces.
2. Description of the Related Art
Generally, an electronic circuit utilizing differential signals (normal-mode signals) includes two differential lines. Due to various reasons, common-mode noise (common-mode signal), which causes radiation of electromagnetic noise, disadvantageously flows in these differential lines. Accordingly, a common-mode choke coil which defines a noise filter is connected to a mid-portion of the differential lines so as to permit the normal-mode signal to pass through and to reflect the common-mode signal, thereby eliminating common-mode noise.
In the above-described related art, noise is suppressed by reflection loss. Accordingly, if a noise filter is disposed in a mid-portion of the lines that connect the circuits, noise having a specific frequency often resonates between the noise filter and a peripheral circuit, which increases the noise despite the noise filter.
The signal frequency used in digital devices is increasing, and there is an increased number of electronic devices using signal frequencies of at least 100 MHz. Thus, the frequency of common-mode noise is also increasing, and the line length between the noise filter and a peripheral component or the line length between a plurality of components is vulnerable to the noise. Accordingly, in known noise filters, noise cannot be sufficiently eliminated due to the influence of the resonant frequency caused by the reflection, and signal waveforms are distorted. Thus, in electronic devices using high frequency signals, noise filters that eliminate noise by utilizing reflection loss cannot be effectively used.
There is a noise filter which includes a chip coil in which two lines are embedded in a medium, for example, in ferrite. In this case, if the attenuation (permeability) ratio of one of the common-mode signal and the normal-mode signal is set, the attenuation ratio of the other mode signal is also set since the two lines are disposed in the medium, which is uniform. It is thus difficult to set the attenuation ratio for each of the mode signals.